Research

My research group builds AI systems that optimize real-world business objectives, with a particular emphasis on trustworthy and responsible learning methods. We take an interdisciplinary approach spanning several fields including machine learning, operations research, and statistics.

AI models pre-trained on internet data can understand text, code, audio, and video. However, as public data sources become exhausted, it is evident that enabling applications beyond consumer chatbots requires a thoughtful approach to data curation. Mistakes are costly in business applications, and intelligent agents must carefully collect and leverage proprietary data, such as customer feedback and user interactions.

My research group develops trustworthy AI-driven decision-making systems that optimize long-term outcomes. In particular, we take a holistic “process view” of AI systems.

Process view of AI (not just a single model): Methodological development in ML largely focuses on model training. Taking a system-level view, we identify central bottlenecks in AI systems and resolve them by building computational and data-centric foundations.

Trustworthy AI

Modern data collection systems acquire data from heterogeneous sources, and classical approaches that optimize average-case performance yield brittle AI systems. They fail to i) make good predictions on underrepresented groups, ii) generalize to new environments, even those similar to that seen during training, and iii) be robust to adversarial examples and long-tailed inputs. Yes, even the largest models trained on the entirety of the internet! Despite recent successes, lack of understanding on the failure modes of AI systems highlights the need for models that i) reliably work and ii) rigorous evaluation schemes and diagnostics that maintain their quality.

Language for distribution shifts

Different distribution shifts require different solutions. Understanding why model performance worsened is a fundamental step for informing subsequent methodological and operational interventions. Heterogeneity in data helps robustness, but the cost of data collection is often a binding constraint. We build a nuanced modeling language for quantifying data heterogeneity (or lack thereof), and use it to make optimally allocate limited resources in the AI production pipeline. To learn more, watch the following NeurIPS tutorial and take a look at the following two papers.

Trustworthy AI
Diagnosing Model Performance Under Distribution Shift

Tiffany Cai, Hongseok Namkoong, and Steve Yadlowsky

arXiv:2303.02011 [stat.ML], 2023

Second round review in Operations Research; Conference version appeared Symposium on Foundations of Responsible Computing 2023

Abs arXiv Bib Video Code Slides

Recent advances in AI present significant opportunities to rethink the design of service systems with AI at the forefront. Even in the era of LLMs, managing a workforce of human agents (“servers”) is a crit- ical problem. Crowdsourcing workers are vital for aligning LLMs with human values (e.g., ChatGPT) and in many domains, the cost of human annotation is a binding constraint (e.g., medical diagnosis from radiologists). This work models and analyzes modern service systems involving human reviewers and state-of-the-art AI models. A key intellectual challenge in managing con- gestion within such service systems is endogeneity. Prediction is never the goal, and the link between predictive performance and downstream decision-making performance is not straightforward due to endogeneity. Our work crystallizes how classical tools from queueing theory provide managerial insights into the design of AI-based service systems.
@article{CaiNaYa23, title = {Diagnosing Model Performance Under Distribution Shift}, author = {Cai, Tiffany and Namkoong, Hongseok and Yadlowsky, Steve}, year = {2023}, journal = {arXiv:2303.02011 [stat.ML]}, note = {Second round review in Operations Research; Conference version appeared Symposium on Foundations of Responsible Computing 2023}, url = {https://arxiv.org/abs/2303.02011}, }

Trustworthy AI
On the Need for a Language Describing Distribution Shifts: Illustrations on Tabular Datasets

Jiashuo Liu*, Tianyu Wang*, Peng Cui, and Hongseok Namkoong

arXiv:2307.05284 [cs.LG], 2024

Conference version appeared in NeurIPS 2023.

Abs arXiv Bib Video Slides Website

Different distribution shifts require different interventions, and algorithms must be grounded in the specific shifts they address. Advocating for an inductive approach to research on distributional robustness, we build an empirical testbed, "WhyShift", comprising of natural shifts across 5 tabular datasets and 60,000 model configurations encompassing imbalanced learning algorithms and distributionally robust optimization (DRO) methods. We find Y|X-shifts are most prevalent on our testbed, in stark contrast to the heavy focus on X (covariate)-shifts in the ML literature. We conduct an in-depth empirical analysis of DRO methods and find that the underlying model class (e.g., neural networks, XGBoost) and hyperparameter selection have a first-order impact in practice despite being overlooked by DRO researchers. To further bridge that gap between methodological research and practice, we design case studies that illustrate how such a refined understanding of distribution shifts can enhance both data-centric and algorithmic interventions.
@article{LiuWaCuNa24, title = {On the Need for a Language Describing Distribution Shifts: Illustrations on Tabular Datasets}, author = {Liu$*$, Jiashuo and Wang$*$, Tianyu and Cui, Peng and Namkoong, Hongseok}, year = {2024}, journal = {arXiv:2307.05284 [cs.LG]}, url = {https://arxiv.org/abs/2307.05284}, note = {Conference version appeared in NeurIPS 2023.}, }

Foundations of distributional robustness

Our vision is to build robust and reliable learning procedures that make decisions with a guaranteed level of performance over its inputs. My Ph.D. thesis built the statistical, and computational foundations of robust machine learning. As robustness is a central topic spanning across multiple fields, my subsequent works have developed robust algorithms for deep learning , causal inference, reinforcement learning, and safety evaluation of autonomous vehicles. These works have led to new approaches toward fairness by characterizing fundamental connections between robustness and fairness. Watch my talk (@ Google Brain, 2021) and the following two representative papers to learn more.

Trustworthy AI

Learning Models with Uniform Performance via Distributionally Robust Optimization

John C. Duchi, and Hongseok Namkoong

Annals of Statistics, 2021

arXiv Bib Video Code Slides

@article{DuchiNa21,
  author = {Duchi, John C. and Namkoong, Hongseok},
  title = {Learning Models with Uniform Performance via Distributionally
                    Robust Optimization},
  year = {2021},
  volume = {49},
  number = {3},
  pages = {1378-1406},
  journal = {Annals of Statistics},
  url = {https://projecteuclid.org/journals/annals-of-statistics/volume-49/issue-3/Learning-models-with-uniform-performance-via-distributionally-robust-optimization/10.1214/20-AOS2004.full},
}

Trustworthy AI

Distributionally Robust Losses Against Mixture Covariate Shifts

John C. Duchi, Tatsunori Hashimoto, and Hongseok Namkoong

Operations Research, 2022

arXiv Bib Video Code

@article{DuchiHaNa22,
  author = {Duchi, John C. and Hashimoto, Tatsunori and Namkoong, Hongseok},
  title = {Distributionally Robust Losses Against Mixture Covariate Shifts},
  year = {2022},
  journal = {Operations Research},
  url = {https://pubsonline.informs.org/doi/10.1287/opre.2022.2363},
}

AI-driven decisions

Prediction is never the final goal. To align AI-models optimized to predict short-term (e.g., clicks) with downstream long-term business outcomes (e.g., user utility), we design scalable computational frameworks for learning operational decisions. We derive algorithms from mathematical principles, but test them using rigorous empirical benchmarking practrices rather than relying on theoretical guarantees in idealized, contrived settings.

Adaptive experimentation at scale

Experimentation is the foundation of scientific decision-making. Adaptive methods can significantly improve data efficiency, but standard algorithms are primarily designed to satisfy good upper bounds on their performance (regret bounds) and do not model important operational constraints and are challenging to implement due to infrastructural/organizational difficulties.

We focus on underpowered, short-horizon, and large-batch problems that typically arise in practice. Instead of the typical theory-driven paradigm, we use PyTorch and empirical benchmarking for algorithm development.

Our main observation is that normal approximations, which are universal in statistical inference, can also guide the design of adaptive algorithms. Using large batch normal approximations, we derive an MDP formulation that optimizes instance-specific constants, instead of relying on regret bounds that only hold in large horizons.

By using standard computational tools, our adaptive designs significantly outperform even Bayesian bandit algorithms (e.g., Thompson sampling) that require full distributional knowledge of individual rewards.

AI-driven Decisions
Adaptive Experimentation at Scale: A Computational Framework for Flexible Batches

Ethan Che, and Hongseok Namkoong

arXiv:2303.11582 [cs.LG], 2023

Major revision in Operations Research

Abs arXiv Bib Video Slides Website

Starting with my one-year stint at Meta’s adaptive experimentation team, I’ve been pondering on how bandit algorithms are largely designed by theoreticians to achieve good regret bounds and are rarely used in practice due to the difficulty of implementation and poor empirical performance. In this work, we focus on underpowered, short-horizon, and large-batch problems that typically arise in practice. We use large batch normal approximations to derive an MDP formulation for deriving the optimal adaptive design. Our formulation allows the use of computational tools for designing adaptive algorithms, a break from the existing theory-driven paradigm. Our approach significantly improves statistical power over standard methods, even when compared to Bayesian bandit algorithms (e.g., Thompson sampling) that require full distributional knowledge of individual rewards. Overall, we expand the scope of adaptive experimentation to settings that are difficult for standard methods, involving limited adaptivity, low signal-to-noise ratio, and unknown reward distributions.
@article{CheNa23, title = {Adaptive Experimentation at Scale: A Computational Framework for Flexible Batches}, author = {Che, Ethan and Namkoong, Hongseok}, year = {2023}, journal = {arXiv:2303.11582 [cs.LG]}, note = {Major revision in Operations Research}, url = {https://arxiv.org/abs/2303.11582}, }

Online decision-making with scalable ML tools

Real-world decision-making requires grappling with a perpetual lack of data as environments change. Intelligent agents must comprehend uncertainty and actively gather information to resolve it. Leveraging scalable computational tools in ML—neural networks, SGD based on auto-differentiation, validation losses—to balance exploration and exploitation is a longstanding challenge.

Consider cold-start problem on recommender systems, where an online platform must learn high-quality items among a large catalogue. The standard ML approach uses item features to predict engagement—the de facto industry standard. Instead, we propose a sequence modeling approach where we predict the sequence of engagements observed for the item using an autoregressive model (e.g., transformer).

Main insight: Recommending items based on autoregressively imputed observations implements Thompson sampling that uses prior learned over vast historical data. Training good autoregressive sequence models thus allows you to solve complex online decision-making problems!

AI-driven Decisions

Posterior Sampling via Autoregressive Generation

Kelly Zhang*, Tiffany Cai*, Hongseok Namkoong, and Daniel Russo

arXiv:2405.19466 [cs.LG], 2024

Selected for presentation at the Econometric Society Interdisciplinary Frontiers: Economics and AI+ML conference

arXiv Bib Slides

@article{ZhangCaNaRu24,
  title = {Posterior Sampling via Autoregressive Generation},
  author = {Zhang$*$, Kelly and Cai$*$, Tiffany and Namkoong, Hongseok and Russo, Daniel},
  journal = {arXiv:2405.19466 [cs.LG]},
  year = {2024},
  note = {Selected for presentation at the Econometric Society
                    Interdisciplinary Frontiers: Economics and AI+ML
                    conference},
  url = {https://arxiv.org/abs/2405.19466},
}

AI-based service systems

Recent advances in AI present significant opportunities to rethink the design of service systems with AI at the forefront. Endogeneity presents a key intellectual challenge to managing congestion. Prediction is never the goal, but the link between predictive performance and downstream decision-making performance is not straightforward: prioritizing a job based on predictions impacts the delay of other jobs!

Example of a service system based on state-of-the-art AI models: large-scale content moderation systems in online platforms. AI models help human reviewers prioritize toward violating contents most likely to go viral.

We crystallize how classical tools from queueing theory provide managerial insights into the design and operation of AI-based service systems: i) simple policies with heavy traffic optimality guarantees, ii) novel model selection procedure for prediction models with downstream queueing performance as a central concern, and iii) AI-based triage by trading off predictive performance, hiring costs, and congestion costs.

AI-driven Decisions
Design and Scheduling of an AI-based Queueing System

Jiung Lee, Hongseok Namkoong, and Yibo Zeng

arXiv:2406.06855 [math.OC], 2024

Abs arXiv Bib

Recent advances in AI present significant opportunities to rethink the design of service systems with AI at the forefront. Even in the era of LLMs, managing a workforce of human agents (“servers”) is a crit- ical problem. Crowdsourcing workers are vital for aligning LLMs with human values (e.g., ChatGPT) and in many domains, the cost of human annotation is a binding constraint (e.g., medical diagnosis from radiologists). This work models and analyzes modern service systems involving human reviewers and state-of-the-art AI models. A key intellectual challenge in managing con- gestion within such service systems is endogeneity. Prediction is never the goal, and the link between predictive performance and downstream decision-making performance is not straightforward due to endogeneity. Our work crystallizes how classical tools from queueing theory provide managerial insights into the design of AI-based service systems.
@article{LeeNaZe24, title = {Design and Scheduling of an AI-based Queueing System}, author = {Lee, Jiung and Namkoong, Hongseok and Zeng, Yibo}, year = {2024}, journal = {arXiv:2406.06855 [math.OC]}, url = {https://arxiv.org/abs/2406.06855} }

Robust causality

Off-policy methods can learn sequential decision policies using the rich reservoir of previously collected (non-experimental / observational) data. While prediction models can be easily evaluated on previously collected data, assessing decision-making performance requires counterfactual reasoning. Traditional modeling assumptions that allow adjusting prediction models to learn counterfactuals rarely hold in practice. The growth in the nominal volume of data is no panacea: observed data typically only covers a portion of the state-action space, posing challenges in counterfactual learning. Concomitant to unseen data sparsity, shifts in the data distribution are common. Observed decisions depend on unrecorded confounders, and learning good policies requires causal reasoning. Marginalized demographic groups are severely underrepresented; for example, among 10000+ cancer clinical trials the National Cancer Institute funds, fewer than 5% of participants were non-white.

Our existing statistical language falls woefully short as it relies on unverifiable (and often false) assumptions, and we lack diagnostics that can identify failure modes. We develop data analysis tools that can guarantee robust scientific findings and perhaps more importantly, fail in expected ways by highlighting the fundamental epistemic uncertainty in the data.

External validity

While large-scale randomized studies offer a “gold standard” for internal validity, their external validity can be called into question over spatiotemporal changes in the population, particularly when the treatment effect is heterogeneous across the population. To assess and improve external validity, we develop sensitivity analysis frameworks that allows researchers to assess the extent to which existing experiments inform the treatment effect in a new target site and quantify an expected range of the policy effect for each new site.

Robust Causality

Assessing External Validity via Worst-case Subpopulation Treatment Effects

Sookyo Jeong, and Hongseok Namkoong

arXiv:2007.02411 [stat.ML], 2022

Short version appeared in Conference on Learning Theory 2020

arXiv Bib Video Code Slides

@article{JeongNa22,
  title = {Assessing External Validity via Worst-case Subpopulation Treatment Effects},
  author = {Jeong, Sookyo and Namkoong, Hongseok},
  journal = {arXiv:2007.02411 [stat.ML]},
  year = {2022},
  note = {Short version appeared in Conference on Learning Theory 2020},
  url = {https://arxiv.org/abs/2007.02411},
}

Unobserved confounding

Off-policy methods can learn decision policies using the rich reservoir of previously collected (observational) data. A universal assumption that enable counterfactual reasoning requires observed decisions do not depend on any unrecorded confounders that simultaneously affect future states/rewards. This condition is frequently violated in medicine, e-commerce, and public policy, e.g., emergency department patients often do not have an existing record in the hospital’s electronic health system, leaving essential patient-specific information unobserved in subsequent counterfactual analysis.

In the presence of unobserved confounding, even with large samples, it is impossible to precisely estimate the performance of the evaluation policy. To guard against spurious counterfactual evaluations, we propose a worst-case approach where we first posit a realistic notion of bounded unobserved confounding that limits the influence of unrecorded variables on observed decisions and develop corresponding worst-case bounds on the reward.

Robust Causality

Bounds on the Conditional and Average Treatment Effect with Unobserved Confounding Factors

Steve Yadlowsky, Hongseok Namkoong, Sanjay Basu, John Duchi, and 1 more author

Annals of Statistics, 2022

arXiv Bib Video

@article{YadlowskyNaBaDuTi22,
  title = {Bounds on the Conditional and Average Treatment Effect 
                    with Unobserved Confounding Factors},
  author = {Yadlowsky, Steve and Namkoong, Hongseok and Basu, Sanjay and Duchi, John and Tian, Lu},
  journal = {Annals of Statistics},
  volume = {50},
  number = {5},
  pages = {2587--2615},
  year = {2022},
  url = {https://projecteuclid.org/journals/annals-of-statistics/volume-50/issue-5/Bounds-on-the-conditional-and-average-treatment-effect-with-unobserved/10.1214/22-AOS2195.full},
  slide = {YadlowskyNaBaDuTi22-slides.pdf}
}

Research philosophy and methodology

While theoretical insights can provide invaluable principles, their successful operationalization requires recognizing and internalizing the limitations of crude approximations and unverifiable assumptions we put in place for mathematical convenience. My group’s research methodology aims to connect two disparate yet complementary worldviews:

computational tools and mathematical insights from statistical learning, optimization, applied probability, and casual inference
rigorous empirical benchmarking practices arising from the AI research community’s data-centric approach.

I take inspiration from Von Neumann’s perspective on mathematical sciences, which I paraphrase below:

As a mathematical discipline travels far from its empirical source only indirectly inspired from ideas coming from 'reality', it is beset with grave dangers that it will develop along the line of least resistance and become more and more purely aestheticizing. This need not be bad if the discipline is under the influence of researchers with an exceptionally well-developed taste, but the only general remedy is the rejuvenating return to the source: the reinjection of directly empirical ideas. I am convinced that this is a necessary condition to conserve the freshness and the vitality of the subject, and that this will remain so in the future.

I am fortunate to be able to learn from the well-developed taste of my colleagues. Concurrent to this personal education, I (try to) inject empirical ideas to formulate research directions to increase the impact of my research.